High speed digital transmission systems have been highly developed and refined to communicate voice, data and other information. The transmission of video signals is also a highly developed technology, both as to analog and digital signals. The transmission of a video image, whether it be still or moving, requires the communication of a substantial amount of information. Present technology is available for transmitting relatively few channels of video information without being unduly complicated or expensive. The digitization of analog signals, including video signals, is a well developed technology. Indeed, digital compression techniques have been developed to compress the digital signals into much lower bandwidths to optimize the usage of the transmission systems. The compression of digital video signals allows for the low cost and efficient transmission of video information on standard digital transmission systems. Such transmission systems can carry voice and data signals, as well as video signals on the same transmission system, without requiring larger or different types of systems.
The transmission of digital signals by way of optical signals is an emerging technology, primarily due to the high speed transmission medium. Moreover, the fiber optic transmission of digital signals has certain inherent advantages as compared to the standard copper twisted pair or coaxial cable counterparts, such as longer life, longer transmission distances without the use of repeaters or regenerators, and much less susceptibility to electromagnetic interference. While the digitalization and compression of data introduces a known amount of signal degradation, the transmission of the compressed digital signals on a fiber optic system adds no further degradation. Indeed, the distance and number of drops in a fiber optic system no longer affect signal quality. This allows video signals to be monitored anywhere in a transmission network without affecting the quality of the video signal. Video signals can thus be more effectively monitored in management or administrative offices as well as at dedicated monitoring stations.
The new video compression standards such as JPEG and MPEG, and others, allow for full motion video to be transmitted in bandwidths as low as 384 kbits/s. Bandwidth requirements affect video quality, with higher bandwidths providing better quality. Relatively high quality video can now be transmitted in bandwidths as low as 2 Mbits/s. These signals are easily carried on standard transmission protocols such as DS-1, E1, frame relay and ATM. Video signals carried by the public communication network allow a camera to be monitored from a remote location anywhere in the world where there is access to a reliable public communication network.
The Synchronous Optical NETwork (SONET) transmission system is being incorporated into all of the major public telecommunication transmission networks on a world-wide basis. In Europe, the corresponding SONET system is known as the Synchronous Digital Hierarchy (SDH) system. The SONET transmission of optical signals is carried out according to a well-known protocol. With the availability of efficient video compression techniques and the world digital transmission standards of SONET/SDH, video information can be distributed in a manner never before considered by the video industry. These new methods are brought about because of low cost digitalization and compression of the previously high bandwidth analog signals. The low cost compression coupled with new switching capabilities of SONET/SDH now allow a switch base distribution system for video signals.
Current video distribution or transmission can be classified into two primary groups, namely entertainment distribution and video monitoring. The entertainment distribution of video information includes normal broadcast and cable television systems where there are a relative small number of video channels and a large number of monitors or subscribers. The video monitoring field includes the security industry, where a large number of camera or video channels are monitored by a relatively few number of monitors or subscribers. The term security is used in a broad sense to include any remote monitoring for obtaining security or operational information. The video monitoring field includes the remote monitoring of power distribution systems and pipelines, the remote monitoring of air, rail and highway transportation system, and the remote monitoring of large facilities such as airports, prisons, manufacturing facilities, campus environments, government facilities, etc.
FIG. 1 illustrates apparatus that is well known in the video distribution field for switching the video signals of a large number of cameras to a smaller number of video monitors. A plurality of video cameras 10 and associated analog-to-digital converters (not shown) couple digitized signals by way of respective T1 carrier lines 20 to a respective node on a counter-rotating optical network 24. The digitized NTSC signals from each video camera 10 are compressed and coupled by the respective node 22 onto both optical fibers 24a and 24b according to the SONET protocol. The optical fiber 24a is duplicated, in that the same information is carried according to the same protocol on the optical fiber 24b, but in an opposite direction. The counter-rotating nature of the optical network shown in FIG. 1 enhances reliability of the system. Each node 22, 26 and 28 may include many video cameras and associated interfaces, as well as data and digitized voice communication capabilities. A base node 30 couples the video signals via a large video switch matrix 36 to a number of monitors. Moreover, while the system of FIG. 1 illustrates four nodes, there may be fewer or many more nodes, each separated geographically up to 100 miles, or so.
The SONET protocol can operate in different modes. The OC-1 mode includes a frame having a total of 810 bytes (e.g., channels) of data, 27 of which are overhead, and the remaining 783 bytes can be used as payload or data. The cyclic period of each OC-1 SONET frame is 125 microseconds. The OC-1 mode is the most typical operating mode, and operates at 51.84 Mbit/s. An OC-3 mode operates at 155.52 Mbit/s; an OC-12 mode operates at 622.08 Mbit/s; and an OC-48 mode operates at 2488.32 Mbit/s. As can be appreciated, with higher optical carrier operating speeds, much more data can be transmitted per unit of time, but the cost and complexity of the equipment increases accordingly. The SONET protocol is defined in explicit detail in Bellcore Specifications Synchronous Optical Network (SONET) Transport Systems; Common Generic Criteria", Document No. GR-253-CORE, Issue 2, December 1995; and TR-NWT-000496 SONET Add/Drop Multiplex Equipment (SONET ADM), Generic Criteria, Issue 3, May 1992, the disclosure of which is incorporated herein by reference in its entirety.
The video distribution system of FIG. 1 includes a base node 30 having circuits 32 for retrieving the video data from the various SONET VT channels. The video signals are decompressed by the circuit 32 and transferred as basic NTSC video signals on respective coaxial lines 34 to a switching matrix 36. With this arrangement, if the video distribution system had 100 video cameras 10, there would be 100 video cables 34 continuously coupling 100 channels of video signals to the matrix 36. The matrix 36 functions to switch a fewer number of monitors 38 to selectively present the video information on any of the matrix inputs 34. The lines 40 coupling the video monitors 38 to the switching matrix 36 are standard coaxial-type cables. A switch panel 42 is coupled to a computer 44 which, in turn, is coupled by an RS-232 line 46 to the switching matrix 38. The switching matrix 36 includes circuits for decoding the signals on the RS-232 line 46 and for connecting any one monitor 38 to a specific video input 34 of the switch matrix 36. The switch panel 42 may be buttons, switches or a desktop computer and associated mouse, or other equipment.
The conventional video distribution system shown in FIG. 1 is wasteful of bandwidth, in that the video signals of each camera 10 are coupled as continuous live signals to the input of the switch matrix 36, even if the video signal of a camera is not selected by the switch panel 42 for viewing on a monitor 38. As noted above, if 100 cameras are employed in the video distribution system, then the optical network 24 must be capable of simultaneously handling the bandwidth for all 100 channels of video signals, even if only 10 monitors 38 are usable at one time. This type of system is commonly termed a point-to-point system, or a "home run" system, where all the video signals are carried from all cameras to the base node 30. As can be appreciated, the transmission of video signals, even though compressed, requires substantial bandwidth, as compared to other data and digitized voice information. In view that in most transmission systems the bandwidth is limited, the transmission medium becomes quickly and fully utilized, thereby either blocks the communication of other information, or requires long waiting periods for idle times of the transmission medium. It can be seen from the foregoing that a need exists for a technique that reduces the bandwidth requirements of a video distribution system, without limiting the number or flexibility of video cameras utilized therein. Another need exists for a video distribution system in which the bandwidth requirements are dependent upon the number of video monitors utilized, rather than on the number of cameras employed. Yet another need exists for a technique for activating transmission of video signals of each video camera at its node, only when it is selected at the remote monitor control center.